Method of producing sodium selenate



Nov 1, 1949.

c. W. CLARK Er Al. l METHOD oFrRoDUcING soDIUM SELENATE Filed Jan. 8, .li-347l IHHHUIN INVENToRs harles 7?." Llarl .ibiza 5c/aldea HTTDRNEY Patented Nov. 1, 1949 RIETHOD OF PRODUCING SODIUM SELENATE Charles William Clark, Westmount, Ontario, and

John Henry Schloen, Montreal, Quebec, Canada, assignors" to Canadian Copper Refiners Limited, Montreal, Quebec, Canada, a corporation of Canada Application January 8, 1947, Serial No. 720,822

1 Claim.

Our present invention relates to a method of producing alkali selenates, such as sodium selenate, electrochemically, i. e., by electrolysis, and more particularly for producing compounds of this class in an emcient manner, with a minimum of expense and with a minimum of danger and discomfort to the operators of the process.

Sodium selenate, as an example of compounds of the class to be produced in accordance with the present invention, has been made previously by chemical methods. It has been produced by the oxidation of sodium selenite or selenium dioxide. It has also'been produced by the oxidation of various selenites or by the oxidation of selenious acid to selenic acid, followed by neutralization with caustic soda or soda ash.

Other alkali selenates, such as potassium, lithium, rubidium and cesium selenates, may be produced by similar chemical methods.

These chemical methods are generally undesirable due to the high cost of chemicals and in some instances to the relatively low efciency of the process.

Oxidation of sodium selenite to sodium selenate has also been done experimentally in the prior art by electrolysis but only by a relatively ineicient method giving extremely indifferent results. If sodium selenite be electrolyzed to convert it to sodium selenate by the method referred to without using a diaphragm, only about twothirds of the selenite is so oxidized to selenate at the anode, the other third being reduced at the cathode to sodium selenide, which in turn dissolves in the electrolyte and becomes oxidized, resulting in the undesirable formation of elemental selenium. This results in practice in a very ineiicient process, as the selenium thus produced and deposited on the cathode acts as a high resistance barrier to passage of the electric current to or from the cathode. When the electrolyte is initially sodium selenite, no hydrogen selenide is formed which makes this method less undesirable than when a selenious acid electrolyte is used.

A method has been devised by the prior art of producing selenic acid from selenious acid by electrolysis using a cell having a permeable diaphragm separating the anolyte from the catholyte, the latter being 5normal nitric acid, and using anodes of platinum or lead foil and cathodes of platinum. Despite the use of the diaphragm, some diffusion of the electrolytes takes place, so that there is a partial reduction of selenious acid at the cathode. In addition, the cost of construction of the cell and operation of this process is extremely high and the process consequently not commercial. This method is also undesirable from the commercial point of View as normally only two-thirds of the selenious acid can be oxidized at the anode. The other third is reduced to elemental selenium at the cathode. In this process a secondary reaction occurs by reason of the hydrogen normally produced at the cathode combining with part of the selenium at the catho-de to form hydrogen selehide-an extremely toxic gas. Formation of this gas not only results in a loss of selenium, but also because of its toxicity, makes the operation dangerous to the operators.

A principalobject of the present invention is to provide a commercial method for the electrolytic production of alkali selenates, such as sodium selenate, in a simple cell without a diaphragm, wherein the anode and cathode are base metals, such as lead or lead alloys, and wherein high efficiency is attained without production of hydrogen selenide and with substantially no production of elemental selenium at the cathode.

In attaining this result one or more addition agents are added to the electrolyte in relatively small amounts to prevent the undesired cathode reactions heretofore present in cells of this kind as aforesaid. We have found that when using a slightly alkaline solution preferably having a relatively high initial concentration of sodium selenite, and having a relatively small portion of one or more of the addition agents as hereinafter particularly set forth, the various desirable results above referred to are attained.

Following the electrolysis the sodium selenate dissolved in the solution may be recovered by any recognized process, such as crystallization.

We have also found that acidic electrolytes from pH 4 to 6.9 may be used in our process without production of hydrogen selenide and with substantially no production of elemental selenium at the cathode, but in such case reduction of the addition agent takes place and the 'current elciency is appreciable lower than when using the preferable slightly alkaline electrolytes.

On the other hand by using such acidic electrolytes and by subsequently neutralizing and precipitating the reduced addition agent with soda ash or caustic soda and removing the precipitate by filtration or any other method of mechanical separation almost colorless sodium selenate in crystal form free of the addition agent will be produced.

The various elements of our process and the requirements thereof will now be set forth in detail.

Composition of the electrolyte In the making of a given alkali salt of selenic acid, the initial electrolyte is an aqueous solution of the corresponding selenious salt. For example, Iin 1the making of sodium Sselenate (Na2SeO4), sodium selenite (NazSeOz.) is used in the initial electrolyte. It is` contemplated that concentrations from about 100 to about 450 grams per liter will be employed,.with=a preferred concentration of about 400 grams per liter. Higher concentrations within the limits given are desirable for the reasons 'that `smaller 'cellsand less solution are required andamorelfiiglily concentrated product is obtained, lthusllowering the cost of production of the nal desired product.

As addition agents, it has been found that soluble salts, such as dichr'ornates, 'orthovana-f' dates and molybdates, may be advantageouslyv employed. We also contemplate as desirable the use of` mixtures of two or more of such addition agents. '-Sudh/salts are preferablyemploye -n `t'o'tal concentrations proportional to that of the initial Aconcentration cf the vs'elen'ite used. We .contemplate the use ofia soluble dichromate, such as sodium dichromate '(NaCrzO1),"in 'con- .centrations of about '1 to about 'Bgram's per liter. `We also contemplate the use of a soluble orthovanadate, ysuch as sodium` orthovanadate (NaVOi), .the equivalent concentration limits 'as .to this material -being from `about"2to about .'1'0 .grams per liter. We have found feasible and contemplate as desirable 'the use .of a soluble molybdate, such as sodium'molybdate .(.Naz'Mo'OQ in concentrations of from about I5 to about '25 preferred, .as .relatively smaller concentrations are effectiveto 4givesubstantia'lly equivalent results. In all cases, the concentration of the addition agent or agents used Should be substantially .proportional to 'the rinitial concentration .of the -selenite used. 'For example, if Ithel lower limit of sodium .selenite .were .used (`0 `grams .per'lter as givenabove) 'the lower limits .given for each .of Lthe addition agents would be .effective On theotherihand, if the'higherlirnit of '.concentrationof sodiumselenite beusedfi. e., 450 .grams ,per liter, asaforesaid, the'higher .limit of .concentration .of .the addition agent or agents `(whether dichromates, .orthovanadates or molyb- ,r

dates or mixtures of .these) should be `used `therewith. `The highlimitsg'iven for the .concentration of the .addition .agent `or agents are not particularly critical, so .that 4somewhat .more `may be .employed -without .interfering with theefciency of .the process, the result of the use of-suchexcess Qbeingmerely the presence -intlflesolutionof excess .addition agent or agents .with -a .corresponding increase v-in the .expense of the .process 'and without any corresponding-increase in the-.desirability .of the resultsobtained.

It has -been A,found further that the -addition agent-orvagents used -should `be quite pure. iFYor example, vwhen ordinary commercial chemicals of the kindV 'mentioned 'are fem-played, it is desirable, .if :not necessary, '-to ipur-ify 'them .as by rrecrystallizationv before 'using them in the lmanner herein taught.

While the vinitialfelectrolytefmade up a-s-:afore- 'said may vand probably will -b`e :in .La Idesired p'I-l frange, this solutionxsh'ould `have a =pII-o'f 4 '-'to "8, with a 'preferable pI-I range of'lll to 8.0 :for best results. It is preferred, therefore, that the pH be adjusted toa1desired;point,aslby the-addition to the introduction of the solution into the cell as the electrolyte therefor.

Electrolysis conditions The electrolyte prepared as aforesaid is then iintroduced lintoza suitable cell, preferably of the one compartment type, having an anode and a cathode, both in direct contact with such electrolyte. A suitable cell need not necessarily `'have any diaphragm or division between the anode and cathode, but such diaphragm or division, if A`present, will lnot necessarily preclude the operationof lthelprocess, although it may hinder lthat `upf-:ration `going to completion as will be 5 fobviousltoithosefskilled in the art. A simple single compartment oellifs preferred. This cell is provided with an anode and a cathode in such a way that there will be a cathode current density (coordinated by the design and area of the cathode and the total current passing through lthe lcell) of about .l0 -to about 200 amperes -per .square lfoot, with a :prefer-red current `density of =about '.50 amperes per :square foot. The anode icurrent .density should be Y:approximately `onevthird lof the cathode current fdensity and .from =about .3 to 'about I50 amperes per `square foot, with about -15 preferred. Such current -density distribution .may .be attained in fa cell -of 'the type 'more :particularly hereinafter described It has been found, `as ,more specifically ,set .forth hereinafter, that lthe action -of lthe -:addition .agent :or `agents linhibiting undesirable iactions at -the cathode lis .interconnected 1in some \way with the cathode current density, so that 'the combinagrams per liter Of the above -the dichromate is :n 'tion of botH'is 'r'equlxedm'der 'that the 'results contemplated in ,accordance .with lthe present invention l and actually .achieved thereby in v:pracl.tice [may Ybe obtained.

We have .found that -a desirable temperature range is ffromlabout to about-160 `with a lpreferredtemperatureofaboutf1 20 fF. .The limit .-inglfactors .in this irespectare -that if th-eftemperature of the electrolyte is too low, crystalsfoffdec- .ahydrated sodium selenate (NaeseulHzO) lare likely to 'berthrown .out of solution. `On lthe -other hand, #if the Vtemperature -lbe 'too high, -there willfbeltoo muchspray-at the surface'of the elec- .trol-yte.

While various different types and designs =of cells lmay be -eiective :in laccordance 'with the principles aforesaid Ain the ycarrying 'out of the -ielectrolytic *oxidation in -accor'dance with the ,present invention, .we 'use ya particular .ftypeo cell, which has lbeen found completely satisfactory :for .this fpurpose. -Such a -cell is shown in :the raccompanying 'drawing in which:

'Figure :1 :is a plan view Tof a ycell :in yaccordance -with `the present invention, ipazit of 'one :of Ithe .electrodes being broken iaway :to :Show certain portions "of the construction;

Fig. 2 Visalview principally 'in vertical section substantially on the vbro-ken :line :2-2`lof 2Fig. .'l showing ithe `.cell =and `-certain :.of its .interior construction;

Fig. 3 is a view :in transverse *vertical section of the -cell taken substantially 1.onthe .iline 23-3 tof Fig. 2;

:Figfli lis a fragmentary "View .inperspectivelof :a lportion of th'ecell'shownl inthe previousfigures; fand Fig. '5 is a :fragmentary .v'i'ew .in vertical-'section illustrating fa modified-.form fof connection lof one Tof the fan'odes to fthe cell ilining.

Turning znow to the accompanying drawings,

ture including an inner lining I made up preferably of sheet lead, burned together in the usual manner for such structures, this structure being supported inside an outer case generally designated at 2, and which is intended as a diagrammatic representation only of any suitable supporting structure. The lining I is extended over the flange 3 of the outer case for purposes which will hereinafter appear.

At each upper end of the cell is provided a pair of spaced socket members 4 in the form of semicylindrical parts which are welded or otherwise suitably secured to the extended lining. These socket members 4 serve to lsupport and position a pair of pipe members 5 which have suspended therefrom platelike pieces of sheet metal 6 serving as a part of the anode of the cell. Inasmuch as the inside of the cell is also electrically connected through the socket members 4 and the pipe members 5 to the plate members 6, the entire lining I also serves as a part of the anode and thus serves to restrict the anode current density. Electrical connection may be made to the anode by a suitable tab 'I welded or otherwise suitably secured to the lining I, the parts I, 3, 4, 5, S and 'I all being of metal and suitably` secured to or associated with one another as to be in good electrical contact for the ow of a relatively heavy current therethrough.

In Fig. 5 there is shown a modified form of means for connecting the ends of the pipe members 5 to the peripheral portions of thelining l extending over the ange 3 of the outer case. For this purpose the ends of the pipe members 5 may be flattened as indicated at and secured as by nut and bolt means 2| to the edge portion of the lining I and the ange 3. This effects a good low voltage contact between the pipe members 5 and the lining I.

Suitably disposed between the socket members 4 as best shown in Fig. 4 are insulating sup'- porting members 8, which may be of any suitable material, such as glass, to provide electrical insulation between the socket members 4 and the portion of the lining I extending over the ange 3, by which it is supported, on the one hand, and the cathode supported thereby, on the other hand. The cathode in the present case consists of. a sheet of suitable metal 9 depended from a transverse metal bar or pipe I0, the ends of which are adapted to be received in semi-cylindrical cavities I I in the insulating members 8. A suitable connecting member I2 may be secured by .welding or other suitable means to the bar or pipe Il) of the cathode for connection to a source of current. The parts 9, III and I2 all being of metal are suitably secured to or associated with one another as to be in good electrical contact for the flow of a relatively high electrical current therethrough.

Current may be supplied to the cell from any suitable source (not shown) through a positive conductor I3 and a negative conductor I4. suitable rheostat, generally indicated at I5, or other current controlling means may be interposed in one or both of these conductors as shown.

The conductors are connected respectively to the tab 'I' and the connecting member I2 as diagrammatically illustrated in Fig. 2.

In the normal operation of the cell, hydrogen is evolved at the cathode and is released at the electrolyte surface. This gas causes some spray of electrolyte which may be confined to the cell by a glass `cover II and non-conducting supports therefor I6.

' -To provide agitation of the cell electrolyte and thus to prevent segregation of sodium selenite below the lower edge of the electrodes, air at a desired relatively low pressure may be introduced into the cell continuously or at regular intervals through a U-shaped pipe I8 perforated with small holes throughout its length and connected with a suitable air supply line I9. These pipes may be of lead or may be of some non-conducting material such as plastic.

During the electrolysis, the temperatureof the electrolyte may be brought to and maintained at a suiiciently high degree for desirable operating conditions due to the resistance of the electrolyte to the passage of current therethrough and the heat consequently generated by such passage of current. On the other hand, if it be :found desirable at the initial stage or during operations to provide a higher temperature than'is afforded by this passage of current, it is contemplated that the necessary additional heat may be supplied to the electrolyte inany suitable way, not particularly shown in the drawings, for example, by heat externally applied and which flows through the walls of the cell, or alternatively by heat generated within the cell by the introduction there-` into of a conventional type heating element in some manner which will notinterfere with the operations otherwise taking place within the cell. The electrodes of the cell as aforesaid may be made of various materials, for example, lead or steel. The lead used may be pure lead or an alloy thereof, for example, one containing lead alloyed with 6% antimony or 1 to 10% silver. In this connection we have found that any conductor may -be used as the cathode, provided it is substantially not susceptible to attack by either sodium selenite or selenate. Platinum may, of course, be used as either or both electrodes. On the other hand, such matals as stainless steel, nickel, copper and aluminum have been found unsuitable as anodes. It is contemplated that silver or gold or tin or any other base metal l plated with chromium may be suitable as anodes.

In one actual example of the operation of the process, a cell constructed substantially as shown in the drawings and filled with an electrolyte to a depth of 24 inches to give an effective cathode area of 5.5 square feet and an effective anode area of 16.8 square feet was used. The volume of the electrolyte in the cell was 138 liters. In operation, 120 lbs. of sodium selenite was dissolved in warm water to make 138 liters of solution giving a concentration of about 395 grams per liter. T o this solution 700 grams of sodium dichromate was added, giving a concentration of about 5 grams per liter. The pH of the electrolyte was 7.5. The cell was operated at a loadof 300 amperes and had a potential drop of about 4.5 volts. When operated at 200 amperes the potential drop was 3.7 volts. while at amperes the potential drop was 2.7 volts. The temperature of the cell ranged from 100 to 160 F. Water was added as necessary to replace that evaporated by the heat generated during electrolysis, which lis a desirable operating procedure. The cell was constructed of all lead and substantially in accordance with that shown in the accompanying drawings and described hereinabove. In one particular trial, the electrolysis process took place to substantially complete chemical conversion in a time period of about 69 hours with an average current of 260 amperes and a potential drop of 4.3 volts. 'I'here resulted a currenteiciency of about 93% and a power consumption of about 0.59 kwh. per pound ofl sodium selenate produced-` Substantially' all the sodium selenite was converted` to sodium selenate in this time period with no: diiiiculpties being experienced through reduction ofl selenium at the cathode.

After the oxidation of the selenite to selenate has been completed, current flow is stopped, the solution is withdrawn from the cell and' replaced by a fresh batch of solution. as aforesaid. The solution thus withdrawn may be further treated in any suitable way known in the art for the recovery of the selenate as a. desired final. product, for example, by evaporation and crystallization. Mother liquor may be returned tothe evaporator for further crystallization or may be returned to mix with succeeding batches of liquor or solution supplied to the cells.v In this Way substantial amounts of the addition agent or agents may be saved and returned for re-use in the process. Washing the crystals with cold water aids removal of substantially all the addition agentv or agents.

While various theories have been advanced, tending to explain to a greater or less extent the operation of the addition agent or agents in securing the desiredresults as set forth above, no such theory or theories is relied upon in accordance with the present invention. Such results may possibly be explained by one or more of the following: the prevention of polarization at the cathode, prevention ofthe reduction of selenium at the cathode by providing an oxidizing influence at that point, or some catalytic operation taking place. The basis for the theory of catalytic operation is that a substantially pure addition agent is quite eiective as aforesaid, while the same relatively impure chemical is substantially ineffective to attain the desired results. These theories, however, are merely suggested for the purpose of making a full disclosure within the present knowledge of the applicants in this case and not as a basis upon hich the patentability of the appended claims is rested.

While we have shown and described but one principal embodiment of ourinvention and as to the chemical and electrochemical phases thereof have given the limits of the operating ranges insofar as are now known to us, we do not wish to be limited except by the scope of the appended claim, which is to be construed validly as broadly as'the state of the prior art permits.

What is claimed is:

The method of producing sodium selenate by electrolysis without at any time forming ele- 8 mental selenium, which comprises the steps of preparing an aqueous solution of sodium selenite having a concentration of about 400 grams per liter and containing a minor proportion of at least one addition agent, which is selected from the group consisting of sodium dichromate in a concentration of about 5 grams per liter, sodium orthovanadate in a.l concentration of about 10 grams per liter, and sodium molybdate in a concentration of about 25 grams per liter, whereby said addition agent. is present in, a total concentration suicient substantially to prevent reduction of selenium atthecathode upon electrolysis, adjusting the pH ofthe solution to the range of about 7.1 to 8.0; supplying such a` solution to a one-compartment electrolytic cell having an anode and a cathode .both directly incontact with the solution. said anode and said cathode both being formed of metallic material composed predominantly of lead; applying a direct current to the anode and; cathode of said cell in an amount coordinated with thedimensions of said anode and said cathode suchthat there will bean anode current density of about 15 amperes per square foot and a cathode current density of about amperes per square foot, maintaining the temperature of the solutionabout F. during the electrolysis; continui-ng the processv as: aforesaid to complete conversion of the selenite to selenate, and recovering sodium selenate from the solution following electrolysis.

CHARLES WILLIAM' CLARK. JOHN HENRY'SCHLO-EN.

REFERENCES CITED The following-references are of record in the ille of this patent:

UNITED STATES PATENTS Number Name Date 342,646 Simon May 25, 1886 1,125,086 Fuller Jan. 19, 1915 2,145,745 Armstrong et al. Jan. 31, 1939 2,223,929 Lowenstein Dec. 3, 1940 2,375,933 Lowenstein May 15', 1945 OTHER4 REFERENCES Zeitschrift fur' Elektrochemie, vol. '72, (1901), pp. 398-405.

Berichte Deutsche Chemische Gesellschaft, vol. 36, (1904), pp. 4262-4266;

Alien Property Custodian No. 245,310, May 18, 1943. 

